Toast the Peppers: Begin by heating a pan and toasting the ancho, guajillo, and optional chiles de arbol until they darken, unlocking their flavors.
Soak the Peppers: After toasting, steep the peppers in hot water to soften them, preparing for blending.
Sauté Vegetables: In the same pan, cook onions and tomatoes, then add garlic, sautéing until fragrant.
Blend the Sauce: Transfer the sautéed vegetables and softened peppers to a food processor, adding seasonings, vinegar, and beef stock to create the birria sauce.
Marinate the Meat: Coat the lamb shoulder in the sauce, allowing it to marinate and absorb the flavors.
Cook the Birria: Combine the marinated meat, additional roasted tomatoes, and beef stock in a pot, simmering until the meat is tender and flavorful.
Serve: Enjoy the birria as a hearty stew or shred the meat for tacos, accompanied by the rich consome.